NMR Studies of Metal Hydrides in Carbon Scaffolds

Lithium borohydride (LiBH4) and alane (AlH3) are two of the top candidates for solid-state hydrogen storage. They both have a large mass-fraction of potentially reversibly stored H, but the enthalpy change during hydrogen desorption is too high for LiBH4 and too low for AlH3. To meet the Department of Energy requirement for onboard hydrogen storage applications, porous scaffolds have been studied to modify the thermodynamics and kinetics. Confining metal hydrides in these porous scaffolds leads to a large surface interface between the hydride and the scaffold, allowing some control of the interfacial energies between the scaffold and the hydride. In this thesis, I will show the 1H static NMR study of the ionic motions for LiBH4 in porous scaffolds and 15N MAS NMR study of the interaction between the scaffold and infiltrated LiBH4 or AlH3. LiBH4 in porous scaffolds displays a motionally-narrowed fraction of its 1H static NMR spectrum. Here, we report selective inversion experiments to measure the rate of exchange between the mobile and immobile BH_4^- groups. We find the exchange time constant to be nearly temperature independent at ~5 ms. In 15N MAS NMR, there are two species of nitrogen on the porous scaffolds, and the pyridinic nitrogen peak shifts after infiltrated with LiBH4 or AlH3. Pyridinic nitrogen and the infiltrated hydrides may be acting as a Lewis-acid/base complex